1. Field of the Invention
This invention relates to a method of preparing purified silver nitrate, and, more particularly, to a method of removing metal impurities involved in a crude silver nitrate solution in the production of silver nitrate of high purity.
2. Description of the Prior Art
Silver nitrate has been produced heretofore by dissolving metallic silver in nitric acid upon heating. Crude silver nitrate thus produced, however, still contains impurities such as Fe, Cu, Pd, Pb, Ni, Au, Al, Zn, Bi, Hg, Cd, Cr, etc., which must be removed by further purification where a higher degree of purity is required such as in the production of photographic materials, for catalytic use, or for analytical purposes.
To meet such a requirement, various methods have been proposed as for effective removal of metallic contaminants from the crude silver nitrate solution.
One of those methods comprises passing the crude silver nitrate solution through a column filled with activated alumina (Al.sub.2 O.sub.3) or activated magnesia to eliminate metallic impurities (e.g., as disclosed in U.S. Pat. No. 2,614,029, British Pat. No. 629,179, etc.). This method, however, has the drawback that, when the pH of the solution is too low, the alumina in the column dissolves out and contaminates the crude silver nitrate solution even further.
Another method of contaminant removal, which is based on the fact that acetylene or methylacetylene does not react with divalent metals, comprises bubbling gaseous acetylene or methylacetylene into the crude silver nitrate solution to cause a selective reaction with silver, and then separating the Ag.sub.2 C.sub.2 or AgC.sub.2 CH.sub.3 thus formed whereby the contaminants remain in the solution (e.g., as disclosed in U.S. Pat. No. 3,800,030). In this method, the entire quantity of silver must be reacted with acetylene or methylacetylene and converted into Ag.sub.2 C.sub.2 or AgC.sub.2 CH.sub.3, which is then again decomposed by nitric acid to obtain purified silver nitrate. Therefore, the procedures are complicated, demanding almost twice as much nitric acid as other methods. The resulting high manufacturing cost makes this method unapplicable industrially. Moreover, the method has another drawback of high hazards due to the unstable nature of Ag.sub.2 C.sub.2 or AgC.sub.2 CH.sub.3 which explosively decomposes upon heating, friction, or even by the action of light.
Still another purification process is known wherein the crude silver nitrate solution is rendered nearly neutral by the addition of silver oxide to separate metallic impurities as hydroxides which have low solubilities. The hydroxides thus formed can be removed by filtration. Although this process can be practiced easily, and is employed widely, limitations exist on the kinds of metallic impurities which can be removed by this process. For example, Ni, Zn, Pb, Cd, Hg, etc., cannot be removed to a satisfactory extent using this process.
To obviate such an undesirable selectivity, an improved method has been proposed wherein iron compounds such as Fe(NO.sub.3).sub.3, Fe(OH).sub.3 or iron powder are added in order to increase the impurity removal efficiency (e.g., as disclosed in U.S. Pat. No. 3,141,731, British Pat. No. 1,042,159, etc.). Even in this modified process, Ni, Zn or Cd cannot be removed to a satisfactory extent. Moreover, on filtering the separated precipitate, ferric hydroxide not only tends to clog the filter membrane, making filtration quite difficult, but also a long time is required to dissolve ferric hydroxide due to the poor solubility of ferric hydroxide in nitric acid. Furthermore, the ferric compound added has a high possibility of contaminating the silver nitrate, which has been demonstrated by the fact that, when the resulting silver nitrate is used for the production of a photographic material, a spot-like fog occurs on the photographic material.